Field of the Invention
The present disclosure relates to a method for producing a beam shaping holographic optical element by providing a master element and a recording element. Especially, the present disclosure relates also to an arrangement for performing the method, a produced beam shaping holographic optical element and a display device comprising the produced beam shaping holographic optical element.
Discussion of the Related Art
Nowadays, liquid crystal displays are often used in electronic applications. Exemplified applications are mobile devices, game computers, tablet computers, monitors, television devices, advertising panels, and the like. Liquid crystal displays comprise a layer or panel with liquid crystals which can be driven electrically. In particular, the polarisation of the light illuminated by the display can be controlled depending on the voltage applied to the liquid crystals. Since these panels produce no light of their own a backlight unit has to be provided to illuminate the panels.
Thereby, a general concern is to provide a liquid crystal display having a high display quality. Factors of a high display quality are the colour space (Gamut), the homogeneity of the illumination, and the contrast relation. Future liquid crystal displays will also require a good steering quality e.g., for providing improved 3-D applications. Backlight units which enable a liquid crystal display to fulfill these requirements are backlight units comprising a beam shaping holographic optical element. A beam shaping holographic optical element is configured to illuminate a defined area at a defined distance from the beam shaping holographic optical element in a homogenous manner. For instance, the liquid display panel, a lens, a diffusor, or the like may be arranged at the defined area. In particular, by recording an element comprising any suitable recording material with a desired pattern, a beam shaping holographic optical element can be produced. It shall be understood that there are a plurality of further application requiring a beam shaping holographic optical element with good steering qualities, like signal lighting.
A particular important quality feature of a produced beam shaping holographic optical element is the steering quality. The steering quality or steering ability means that the beam shaping holographic optical element is able to reconstruct an identical real image of a diffusor independently of the impact location of a pencil of light that hits the beam shaping holographic optical element whereas the pencil of light emerges from a common source point.
A beam shaping holographic optical element in relation to the present disclosure is in particular a holographic optical element which transforms a spatially and/or directionally confined light source into a homogeneously illuminated area at a certain location in space. An important type of such a light source—but not exclusive in the sense of the present disclosure—is a pencil of intense laser light.
To transform the point-like cross section of this pencil of light for example in a plane rectangle with homogeneous brightness distribution at a certain location in space a beam shaping holographic optical element can be used. Such a beam shaping holographic optical element can work in transmission or in reflection geometry or in edge lit geometry and has to reconstruct the real image of a diffusor at the specified location in space in order to generate there the desired homogeneous brightness distribution.
However, the production of beam shaping holographic optical elements having a high quality and sufficiently geometric dimensions is difficult. One prior art approach and its problems will be described in the following with the aid of FIGS. 1 through 3.
FIG. 1 shows a conventional schematic view of an arrangement for producing a beam shaping holographic optical element.
Generally, to be able to reconstruct a real image with a beam shaping holographic optical element a reconstruction beam has to be the phase conjugate beam of the reference beam used for producing a beam shaping holographic optical element (Gerhard K. Ackermann and Jürgen Eichler “Holography a Practical Approach, Wiley VCH Verlag & Co. KGaA, Weinheim, 2007, Chapter 16. Pages 217-218).
FIG. 1 shows an exemplified recording scheme according to the prior art with an illumination beam 104 and a reference beam 103c. A convergent recording beam 103c is used as a reference beam 103c of the holographic recording setup for the present transmission-type beam shaping holographic optical element. The scheme in FIG. 1 is viewed from the top. After splitting the light beam emitted from a common laser light source by means of a suitable beam splitter, two spatial filters 105 generate the divergent illumination beam 104 for illuminating the object 101, for example a transparent diffusor, and the primarily divergent reference beam 103b. 
After being reflected by the concave mirror 107, the primarily divergent reference beam 103b is transformed into a convergent reference beam 103c with a focus located at position 103d. The translucent object 101 generates the recording object beams 106b by diffraction. The object beams 106b interfere with the convergent reference beam 103c at the recording element 102. Element 102 is the element to be recorded with a desired pattern. By recording the recording element 102, the desired beam shaping holographic optical element is produced.
For reconstruction (see FIG. 2) of the real image 106a of the object 101 from FIG. 1, the (phase conjugated) reconstruction beam 103, diverging from a point in space 103a, is generated for example from a focused laser beam in combination with a spatial filter 105 positioned in this focal point. The beam shaping holographic optical element 102 then forms the diffracted beams 106 which form the real image 106a at its desired position.
A problem of the arrangement according to FIG. 1 is the required dimensions of the concave mirror 107 (or positive lens) configured to generate the converging reference beam 103c. The concave mirror 107 must always have larger lateral dimensions than the lateral dimension of the desired beam shaping holographic optical element 102 and the recording element 102, respectively. Thereby, the shorter the focal length—the distance between focal point 103d and the recording element 102—is, the larger the relative size of the concave mirror 107 (or positive lens) has to be compared to the beam shaping holographic optical element 102.
For instance, if the beam shaping holographic optical element 102 itself already has a large size (for example the size of an electronic display with 10″ diagonal size or larger) the concave mirror 107 (or positive lens) has to be much larger than that. This means that the optical components, like mirrors or lenses, which can form the convergent reference beam, become very large and will be very expensive and/or difficult to manufacture. Other problems are the handling of such large and heavy components, their optical alignment and the stabilization and the footprint of the holographic recording setup.
Moreover, if the numerical aperture—the sine of the half of the opening angle of the convergent recording beam 103c in FIG. 1—or equivalent the field angle of the beam shaping holographic optical element 102 in FIG. 2 becomes very large such kind of focusing mirrors 107 and lenses with the necessary large diameter of the opening aperture and the respectively short focal length are virtually not available. An exemplary description of the meaning of numerical aperture is given in FIG. 3, which is the perspective view of the reference beam part given in FIG. 1. Here the numerical aperture is sin(θ/2) in which the angle θ is measured in the plane that contains the largest diameter of the beam shaping holographic optical element 102.
All these difficulties apply also for reflection type beam shaping holographic optical elements or edge-lit type beam shaping holographic optical elements.
As mentioned above, on the one hand, high numerical aperture focusing mirrors 107 and lenses with the necessary large diameter of the opening aperture and the respectively short focal length are virtually not available, or expensive and extremely difficult to handle in a holographic recording setup.
On the other hand, objectives with a high numerical aperture are readily available and could be very cheap as they are already used in consumer devices which entered the mass market. For example a microscope objective of magnification 63× can have a numerical aperture of 0.75. Similarly an objective lens of a BluRay player pickup head has a numerical aperture of even 0.85. Precisely because these objectives will have a limited diameter of the input aperture of a few mm they are suited to generate a high quality divergent beam with the desired numerical aperture.
If the numerical aperture of the reference beam tends to zero the recording reference beam 103c in FIG. 1 and FIG. 3 tends to be a collimated beam and the mirror 107 (lens) is a collimating mirror (lens) with respect to the diverging beam 103b emerging from the pin hole 105. In this case the collimating mirror (lens) size could be reduced close or to its minimum size which is identical to the size of the beam shaping holographic optical element.
The phase conjugated beam of a collimated beam is also a collimated beam just with a reversed direction of propagation. That means to reconstruct the real image in the case of numerical aperture close to zero, instead of doing the readout with the collimated beam with reversed direction, the recorded hologram could be flipped and readout with the original collimated reference beam.
In WO 93/02372, a recording arrangement and method is described for a transmission type beam shaping holographic optical element. In a first step a master element in form of a master beam shaping holographic optical element is recorded with a collimated reference beam.
In a second step the master beam shaping holographic optical element is flipped and read out with the original reference beam used for recording the beam shaping holographic optical element. The real image which is reconstructed by this procedure is copied with a divergent reference beam into a new recording element. This recording element is arranged between the master beam shaping holographic optical element and the position of the reconstructed real image from the master beam shaping holographic optical element wherein the distance between the master element and the recording element is large. Reason for this position is to prevent the zero order light from the master beam shaping holographic optical element from hitting the holographic recording element used for the copy process. If the recorded or produced beam shaping holographic optical element is reconstructed with the divergent beam used as reference beam for the production or copying process the real image of the master beam shaping holographic optical element is reconstructed. As a collimated reference beam is used to record the master beam shaping holographic optical element and a divergent beam is used to record the copy beam shaping holographic optical element the minimum size of mirrors or lenses to from the reference beam can be achieved with the method of WO 93/02372.
However, the display quality, in particular, the steering ability of the produced beam shaping holographic optical element is low. If the produced beam shaping holographic optical element is hit by a pencil of light it strongly depends on the point of impact of the light whether the total real image is reconstructed or not.